DRAFT
5.6
IMT-2000 OFDMA TDD WMAN
5.6.1
Overview of the set of radio interface FDD/TDD components
5.6.1.1
1
TDD component
5.6.1.1.1 Introduction
The IEEE standard relevant for IMT-2000 OFDMA TDD WMAN, designated as IEEE Std 802.16, is
developed and maintained by the IEEE 802.16 Working Group on Broadband Wireless Access. It is
published by the IEEE Standards Association (IEEE-SA) of the Institute of Electrical and Electronics
Engineers (IEEE).
The radio interface technology specified in IEEE Standard 802.16 is flexible, for use in a wide variety of
applications, operating frequencies, and regulatory environments. IEEE 802.16 includes multiple physical
layer specifications, one of which is known as WirelessMAN-OFDMA. OFDMA TDD WMAN is a special
case of WirelessMAN-OFDMA specifying a particular interoperable radio interface. The component of
OFDMA TDD WMAN defined here operates in TDD mode.
The OFDMA TDD WMAN radio interface comprises the two lowest network layers – the physical layer
(PHY) and the data link control layer (DLC). The lower element of the DLC is the medium access control
layer (MAC); the higher element in the DLC is the logical link control layer (LLC). The PHY is based on
orthogonal frequency division multiple access (OFDMA) suitable for use in a 5 MHz,10 MHz, 7 MHz, or
8.75 MHz channel allocation. The MAC is based on a connection-oriented protocol designed for use in a
point-to-multipoint configuration. It is designed to carry a wide range of packet-switched (typically
IP-based) services while permitting fine and instantaneous control of resource allocation to allow full carrierclass Quality of Service (QoS) differentiation. In addition, a multihop relay (MR) specification details the
functionality of relay stations in conjunction with multihop relay base stations in order to provide additional
coverage or performance advantage. Also, a license-exempt (LE) specification delineates the improved
mechanisms, as policies and MAC enhancements, to enable coexistence among LE systems based on
OFDMA TDD WMAN and to facilitate the coexistence of such systems with primary users.
As a technological evolution of OFDMA TDD WMAN, WirelessMAN-Advanced has been introduced for
providing the performance improvements necessary to support future advanced services and applications for
next generation broadband mobile communications. WirelessMAN-Advanced incorporates innovative
communications technologies such as multi-user MIMO (MU-MIMO), multicarrier operation, and
cooperative communications, as well as femto-cells, self-organizing networks, and relays. It also supports
scalable bandwidth operation up to 20 MHz.
5.6.1.1.2 Radio access network architecture
The OFDMA TDD WMAN radio interface is designed to carry packet-based traffic, including IP. It is
flexible enough to support a variety of higher-layer network architectures for fixed, nomadic, or fully mobile
use, with handover support. It can readily support functionality suitable for generic data as well as
time-critical voice and multimedia services, broadcast and multicast services, and mandated regulatory
services.
The radio interface standard specifies Layers 1 and 2; the specification of the higher network layers is not
included. It offers the advantage of flexibility and openness at the interface between Layers 2 and 3 and it
supports a variety of network infrastructures. The radio interface is compatible with the network
architectures defined in ITU-T Recommendation Q.1701. In particular, a network architecture design to
make optimum use of IEEE Standard 802.16 and the OFDMA TDD WMAN radio interface is described in
the “WiMAX End to End Network Systems Architecture Stage 2-3”, available from the WiMAX Forum1.
The protocol layering is illustrated in Fig. 70. The MAC comprises three sub-layers. The service-specific
convergence sublayer (CS) provides any transformation or mapping of external network data, received
through the CS service access point (SAP), into MAC service data units (SDUs) received by the MAC
1
http://www.wimaxforum.org/technology/documents/.
2
Rec. Error! No text of specified style in document.
common part sublayer (CPS) through the MAC SAP. This includes classifying external network SDUs and
associating them to the proper MAC service flow identifier (SFID) and connection identifier (CID). It may
also include such functions as payload header suppression (PHS). Multiple CS specifications are provided
for interfacing with various protocols. The internal format of the CS payload is unique to the CS, and the
MAC CPS is not required to understand the format of or parse any information from the CS payload.
DRAFT
FIGURE 70
OFDMA TDD WMAN protocol layering, showing service access points (SAPs)
3
4
The MAC CPS provides the core MAC functionality of system access, bandwidth allocation, connection
establishment, and connection maintenance. It receives data from the various CSs, through the MAC SAP,
classified to particular MAC connections.
Quality of service (QoS) is applied to the transmission and scheduling of data over the PHY.
The MAC also contains a separate security sublayer providing authentication, secure key exchange, and
encryption.
Data, PHY control, and statistics are transferred between the MAC CPS and the PHY via the PHY SAP
(which is implementation specific).
The 802.16 devices can include Mobile Stations (MS) or Base Stations (BS). As the 802.16 devices may be
part of a larger network and therefore would require interfacing with entities for management and control
purposes, a Network Control and Management System (NCMS) abstraction has been introduced in this
standard as a “black box” containing these entities. The NCMS abstraction allows the PHY/MAC layers
specified in 802.16 to be independent of the network architecture, the transport network, and the protocols
used at the backend and therefore allows greater flexibility. NCMS logically exists at BS side and MS side of
the radio interface, termed NCMS(BS) and NCMS(MS), respectively. Any necessary inter-BS coordination
is handled through the NCMS(BS).
This specification includes a Control SAP (C-SAP) and Management SAP (M-SAP) that expose control
plane and management plane functions to upper layers. The NCMS uses the C-SAP and M-SAP to interface
with the 802.16 entity. In order to provide correct MAC operation, NCMS shall be present within each MS.
The NCMS is a layer independent entity that may be viewed as a management entity or control entity.
General system management entities can perform functions through NCMS and standard management
protocols can be implemented in the NCMS.
5.6.1.1.2.1
BS and MS Functionality
The system architecture consists of two logical entities, the base station (BS) and the mobile station (MS),
with an optional relay station (RS). The basic architectural assumption is of a base station (BS)
communicating in point-to-multipoint fashion with a number of fixed or mobile stations (MSs). The BS is
connected to an IP-based backhaul network. It controls and allocates the resources in spectrum and time.
Transmissions on the downlink (BS to MS) are divided in both time and frequency (using the multiple
sub-carriers provide by OFDMA) for assigning communications to individual MSs. Transmissions on the
uplink (from MS to BS) take place according to the schedule and in the sub-channels assigned by the BS.
In brief, the BS is responsible for:
–
configuring and updating basic parameters;
–
performing bandwidth allocation for DL (per connection) and UL traffic (per MS) and performing
centralized QoS scheduling, based on the QoS/service parameters and the active resource requests
from the MS;
–
communicating to all MSs, through the maps, the schedule of each frame and supporting other data
and management broadcast and multicast services;
–
transmitting/receiving traffic data and control information as MAC protocol data units (PDUs);
–
performing connection admission control and other connection management functions;
–
providing other MS support services such as ranging, clock synchronization, power control, and
handover.
The MS is responsible for:
–
identifying the BS, obtaining MAC parameters, and joining the network;
–
establishing basic connectivity, setting up additional data and management connections, and
negotiating any optional parameters as needed;
–
generating resource requests for connections that require them, based on the connection profiles and
traffic;
–
receiving broadcast/multicast PDUs and unicast PDUs and forwarding them appropriately;
5
–
making local scheduling decisions based on the current demand and history of resource
requests/grants;
–
transmitting only when instructed by the BS to do so or the MS has some information that qualifies
for transmission in one of the allowed contention slots;
–
unless in sleep mode, receiving all schedule and channel information broadcast by the BS and
obeying all medium access rules;
–
performing initial ranging, maintenance ranging, power control, and other housekeeping functions.
Figure 70 is limited to describing a system including a BS and the MSs with which it communicates.
However, the radio interface also provides specifications to allow handover of an MS from one BS to
another. Such handover would typically occur as a mobile device moves toward an adjacent cell. However, it
might also occur due to system-wide efforts at load balancing.
5.6.1.1.2.2
RS Functionality
Multihop relay (MR) is an optional deployment that may be used to provide additional coverage or
performance advantage in an access network. In MR networks, the BS may be replaced by a multihop relay
BS and one or more relay stations (RSs). Traffic and signaling between the MS and the multihop relay BS
are relayed by the RS, thereby extending the coverage and performance of the system in areas where RSs are
deployed. Each RS is under the supervision of a multihop relay BS. In a system providing more than two
hops, traffic and signaling between an access RS and the multihop relay BS may also be relayed through
intermediate RSs.
5.6.1.1.3 Layer 1: Physical layer (PHY)
The radio interface is a special case of the Wireless MAN-OFDMA air interface specified in § 8.4 of IEEE
Standard 802.16. It uses orthogonal frequency-division multiple access (OFDMA), which is an extension of
orthogonal frequency-division multiplexing (OFDM).
5.6.1.1.3.1
OFDMA technology overview
OFDM divides the channel by frequency into orthogonal sub-carriers. Data to be transmitted is divided into
parallel streams of reduced data rate (and therefore longer symbol duration) and each stream is modulated
and transmitted on a separate sub-carrier. The lengthened symbol duration improves the robustness of
OFDM to delay spread. Furthermore, the introduction of a cyclic prefix (CP) eliminates intersymbol
interference if the CP duration is longer than the channel delay spread.
In a typical OFDM implementation, all of the transmitter’s sub-carriers are, at any given time, addressed to a
single receiver; multiple access is provided solely by TDMA time slotting. OFDMA, however, divides the
sub-carrier set into subsets, known as sub-channels. Each sub-channel can address a different receiver at any
given time. In the downlink, each sub-channel may be intended for a different receiver or group of receivers.
In the uplink, multiple MSs may transmit simultaneously as long as they are assigned different sub-channels.
Sub-carriers are used for three purposes:
–
Data transmission
–
Pilot transmission, for various estimation purposes
–
Null transmission, for guard bands and at DC.
The concept is illustrated in Fig. 71. As indicated, the sub-carriers forming one sub-channel need not be
adjacent.
Sub-channelization is a multiple access technique. It provides OFDMA systems increased scheduling
flexibility and a number of performance advantages, including enhanced scalability and advanced antenna
array processing capabilities.
6
FIGURE 71
OFDMA frequency description, schematically showing three sub-channels
Subchannel 1
Subchannel 2
DC subcarrier
Subchannel 3
Channel
Guard band
Guard band
M.1457-71
5.6.1.1.3.2
OFDMA TDD WMAN physical layer details
The PHY utilizes OFDMA with either 512 sub-carriers in a 5 MHz channel or 1 024 sub-carriers in a 7 MHz
or 10 MHz channel. In addition, 1 024 sub-carriers in a 8.75 MHz channel is also utilized for TDD. The
primitive PHY parameters for TDD mode are listed in Table 10A.
TABLE 10A
OFDMA TDD WMAN primitive PHY parameters, TDD mode
FFT size (NFFT)
System channel bandwidth
(BW)
Sampling frequency (Fs)
512
1 024
1 024
1024
5 MHz
10 MHz
8.75 MHz
7 MHz
5.6 MHz
11.2 MHz
10 MHz
8 MHz
Sub-carrier frequency spacing
(∆f = Fs/NFFT)
10.9375 kHz
9.77 kHz
7.8125 kHz
Useful symbol time (Tb = 1/∆f)
~91.43 µs
~102.4 µs
128 µs
Guard (CP) time (Tg = Tb/8)
~11.43 µs
~12.8 µs
16 µs
OFDMA symbol duration
(Ts = Tb + Tg)
~102.9 µs
~115.2 µs
144 µs
Frame duration
5 ms
5 ms
5 ms
OFDMA symbols per frame
(including TTG and RTG)
~48
~43
~34
OFDMA symbols per frame
(excluding TTG and RTG)
47
42
33
5.6.1.1.3.3
Framing and sub-channelization
In the case of TDD, OFDMA TDD WMAN PHY utilizes a 5 ms TDD frame. The frame includes first
downlink and then uplink sub-frames, divided by time gaps to allow the transceivers to switch between
receive and transmit. The two gaps (TTG and RTG) are both included in the 5 ms duration. The asymmetry
between the uplink and downlink sub-frame durations is configurable on a system-wide basis.
The TDD frame structure is illustrated schematically in Fig. 72A.
The frame is shown in two dimensions. The horizontal dimension represents time, which maps directly into
the OFDM symbol sequence. The vertical dimension represents the list of available logical sub-channels.
This maps into frequency, although only indirectly, since the sub-carriers in a given logical channel are not
arranged in sequential order.
7
FIGURE 72A
Schematic illustration of TDD frame structure
ODFMA symbol number
k + 47
Ranging subchannel
DL burst No. 3
FCH
UL burst No. 1
DL burst No. 4
UL burst No. 3
DL burst No. 5
DL burst No. 2
DL-MAP
UL burst No. 2
Preamble
DL-MAP
FCH
Preamble
Subchannel logical number
s
s+1
s+2
t
k+ 1
DL burst No.1
(carrying the UL MAP)
k
UL burst No. 4
DL burst No. 6
UL burst No. 5
s+L
DL
UL
TTG
RTG
M.1457-72
The frame begins with a preamble for synchronization. The following OFDM symbol contains the frame
control header (FCH) and the downlink map (DL_MAP), transmitted simultaneously on different
sub-channels. The FCH includes frame configuration data. The DL-MAP indicates the use of the DL
sub-frame, in time and sub-channel allocation. The UL-MAP that follows provides similar information for
the uplink, though the allocation is relevant not to the current frame but to a subsequent one, in order to
allow the MS time to prepare an appropriate transmission or in accordance with the UL-MAP.
The construction of the sub-channel from individual sub-carriers is called the permutation. OFDMA TDD
WMAN provides a number of possible permutations. The optimal choice depends on the deployment
scenario and instantaneous circumstances; therefore, the permutation may differ from one ODFM symbol to
the next. The specification supports a sequence of permutation zones in the frame, so that different mobile
stations can be served with different permutations.
5.6.1.1.3.4
Adaptive modulation and coding
OFDMA TDD WMAN supports a variety of modulation and coding alternatives. The control is adaptive and
dynamic, so that the BS may select different options for communicating with different MSs and may order
the MS to alter the choices in order to optimize the trade-off of robustness versus capacity.
The BS selects the modulation from among QPSK, 16-QAM, and 64-QAM. For forward error correction,
Convolutional Coding and Convolutional Turbo Coding with variable code rate and repetition coding are
specified. Block Turbo Code and Low Density Parity Check Code (LDPC) are supported as optional
features.
Data randomization is specified in order to reduce the peak-to-average power ratio. Interleaving is specified
to increase frequency diversity.
5.6.1.1.3.5
Fast feedback and hybrid ARQ
OFDMA TDD WMAN specifies an uplink fast-feedback channel to provide time-critical PHY parameter
data to the BS. Parameters include signal-to-noise ratio, MIMO coefficients, and MIMO configuration
parameters.
Additional UL acknowledgment channels may be allocated by the BS to support hybrid automatic repeat
request (HARQ).
8
5.6.1.1.4 Layer 2: Medium access control layer
The medium access control layer (MAC) functionality controls access to the medium, which in this case is
the radio spectrum. The MAC is also responsible for basic functions such as data encapsulation,
fragmentation, radio resource control, radio link control, error detection and retransmission, QoS, security,
sleep mode, and handover.
Although the radio interface is designed primarily to support a connectionless network layer, such as IP, the
MAC is connection-oriented. All services, including inherently connectionless services, are mapped to a
connection. The connection provides a mechanism for requesting resource allocation, associating QoS and
traffic parameters, transporting and routing data, and all other actions associated with the terms of the
service. A 16-bit connection identifier (CID) is assigned to designate each connection. The MAC uses the
CID to identify all information exchanged between BS and MS, including management and broadcast data.
The CID provides a simple and direct way to differentiate traffic. All MAC-level QoS functions, such as the
classifier and QoS scheduler, use the CID to identify and differentiate traffic in order to maintain the service
level and fairness among connections.
5.6.1.1.4.1
Convergence sub-layer
At the transmitter side, the Convergence sub-layer (CS) is responsible for transforming packet-based
protocol data units from the higher layer protocol into MAC service data unit (SDUs), possibly using
payload header suppression (PHS) to suppress some of the packet headers and reduce the burden of carrying
them over the air. The CS then classifies each MAC SDU, assigning it to a particular connection, and passes
it to the MAC CPS. At the receiver side, the CS is responsible for the inverse operations, including
reassembly of packets into their original format with complete headers.
The CS contains a classification function that determines on which connection a particular packet shall be
carried and which PHS rule applies for that packet. The operation is illustrated in Fig. 73, which shows the
downlink case. Classifier parameters are configured during dynamic service signalling.
FIGURE 73
Classification and CID mapping (downlink)
Upper layer entity (e.g., bridge, router, host)
Upper layer entity (e.g., bridge, router, host)
SDU
SDU
SAP
SAP
CID 1
CID 2
Downlink
classifier
.........
Reconstitution
(e.g., undo PHS)
CID n
(SDU, CID, ...)
(SDU, CID, ...)
SAP
802.16 MAC CPS
BS
SAP
SS
802.16 MAC CPS
M.1457-73
9
5.6.1.1.4.2
MAC common part sub-layer
The MAC common part sub-layer (CPS) is responsible for performing the core MAC functions. It receives
MAC service data units (SDUs) from the CS and encapsulates them in its native MAC PDU format for
transmission over the PHY. The MAC CPS also manages the transport connections and QoS, controlling
access to the radio spectrum by the MSs.
Encapsulation may be as simple as adding necessary information to the SDU. However, the MAC CPS also
has the possibility of dividing a single SDU into multiple fragments before transmission, for reassembly at
the receiving MAC CPS. Fragmentation allows more efficient support of higher layer protocols with
variable-size SDUs, given that the underlying PHY used a fixed frame size. The MAC CPS also has the
complementary option to pack multiple higher layer payloads into a single PDU. Since MAC encapsulation
introduces some fixed overhead per PDU, this can improve the efficiency of carrying small SDUs.
A MAC PDU consists of a six-byte MAC header, a variable-length payload, and an optional cyclic
redundancy check. Four header formats, distinguished by the HT field, are defined. The generic header is
shown in Fig. 74.
MAC PDUs generally contain either MAC management messages or convergence sub-layer data. However,
one header type is reserved for uplink PDUs that contain no payload, conveying their information (such as a
resource request) in the content of the header itself. Additional sub-headers are also defined. For example,
the MS can use the grant management sub-header to convey bandwidth management needs to the BS. The
fragmentation sub-header contains information that indicates the presence and orientation in the payload of
any fragments of SDUs. The packing subheader is used to indicate the packing of multiple SDUs into a
single PDU. The grant management and fragmentation sub-headers may be inserted in MAC PDUs
immediately following the generic header if so indicated by the Type field. The packing sub-header may be
inserted before each MAC SDU if so indicated by the Type field.
FIGURE 74
EKS
(2)
Rsv (1)
CI (1)
Type (6)
ESF = 0 (1)
EC (1)
HT = 0 (1)
Generic MAC header format
LEN LSB (8)
CID MSB (8)
CID LSB (8)
HCS (8)
LEN
MSB (3)
M.1457-74
5.6.1.1.4.2.1
Uplink scheduling services
The scheduling algorithm is not specified in the standard but is critical to efficient multimedia delivery when
the BS supports a variety of disparate connections. The BS is presumed capable of scheduling its own
downlink transmissions based on QoS information developed in the CS. Uplink scheduling is more
complicated because, while resource allocation is under the control of the BS, only the MSs know in real
time their immediate transmission demands.
10
In order to allow efficient QoS-based scheduling, a number of uplink scheduling services are defined, with a
specific service assigned to each connection. The QoS categories are summarized in Table 11.
Resource requests, for transmission slots, are initiated by a specific connection at the MS. However, grants
are allocated not to the connection but to the supporting MS. The MS is required to manage the slots
allocated to it, assigning them to the multiple connections it supports. By distributing the management and
permitting local resource allocation, over-the-air negotiation is minimized and rescheduling decisions are
made more quickly and effectively.
5.6.1.1.4.2.2
Radio link control
As noted in § 5.6.1.3.2, OFDMA TDD WMAN supports adaptive modulation and coding. The MAC CPS is
responsible for radio link control. This involves managing the modulation and coding selection at the MS
through interactive message exchange based on monitoring the ratio of carrier signal to noise and
interference.
TABLE 11
OFDMA TDD WMAN Uplink scheduling services
QoS category
Typical applications
QoS specifications
UGS
Unsolicited grant service
VoIP
Maximum sustained rate
Maximum latency tolerance
Jitter tolerance
rtPS
Real-time packet service
Streaming audio or video
Minimum reserved rate
Maximum sustained rate
Maximum latency tolerance
Traffic priority
ErtPS
Extended real-time packet service
Voice with activity detection (VoIP)
Minimum reserved rate
Maximum sustained rate
Maximum latency tolerance
Jitter tolerance
Traffic priority
nrtPS
Non-real-time packet service
File transfer protocol (FTP)
Minimum reserved rate
Maximum sustained rate
Traffic priority
BE
Best-effort service
Data transfer, web browsing, etc.
Maximum sustained rate
Traffic priority
5.6.1.1.4.2.3
Energy conservation in the MS
The MAC CPS controls two energy-saving modes – Sleep Mode and Idle Mode – to conserve energy in the
MS. During Sleep Mode, the MS observes pre-negotiated periods without transmission. Idle Mode is
intended as a mechanism to allow the MS to become periodically available for DL broadcast messaging
without registration at a specific BS as the MS traverses an air link environment populated by multiple BSs,
typically over a large geographic area.
5.6.1.1.4.2.4
Handover
The MAC CPS supports optimized hard handover.
11
5.6.1.1.4.3
Security sub-layer
The security sub-layer, which operates between the PHY and the MAC CPS, is responsible for providing
strong encryption, decryption, mutual authentication, and secure key exchange. Security is maintained as a
separate sub-layer so that it may be upgraded as necessary. Also, the key functionality internal to the
sub-layer is also modular, to provide easy maintenance upgrade. For example, the protocol provides a means
of identifying one from a set of supported cryptographic suites, each of which specifies data encryption and
authentication algorithms, and the rules for applying those algorithms to a MAC PDU payload.
The security sub-layer utilizes a security association (SA), which is a set of information shared between the
transmitter and receiver. Each SA contains information on the cryptographic suite used for that SA and may
also contain keys, such as the traffic encryption keys (TEKs), along with the key lifetimes and other
associated state information. Prior to transmission, the MAC PDUs are mapped to an SA. The receiver uses
the CID to determine the correct SA and applies the corresponding processing to the received PDU.
Device and user authentication use the IETF EAP protocol. OFDMA TDD WMAN encrypts user data using
the AES-CCM cryptographic suite, with the Advanced Encryption Standard (AES) algorithm in the counter
with CBC-MAC (CCM) mode, with 128-bit keys. The keys are generated using EAP authentication and
managed by a Traffic Encryption Key (TEK) state machine. MAC management messages are AES encrypted
and authenticated. A three-way handshake scheme is supported to optimize re-authentication during
handover.
5.6.1.1.5 Smart antennas
OFDMA TDD WMAN specifies the use of smart antenna technologies, including antenna beamforming,
space-time coding, and spatial multiplexing, which increase the cell size, data throughput, and spectral
efficiency. These techniques reduce the sensitivity of the system to fading and multipath transmission
effects.
5.6.1.1.6 Summary of major technical parameters
TABLE 12A
OFDMA TDD WMAN parameters and capabilities, TDD mode
Value
IEEE 802.16
Subclause
TDD
§ 8.4.4
OFDMA
§ 8.4
Parameter/Capability
Duplex method
Physical layer mode
System channel bandwidth
5 MHz
10 MHz
8.75 MHz
7 MHz
FFT size
512
1 024
1 024
1024
Frame duration
5 ms
5 ms
5 ms
§ 8.4.5.2
Transmit transition gap (TTG)
105.714 µs
87.2 µs
188 µs
§ 8.4.5.2
Receive transition gap (RTG)
60 µs
74.4 µs
60 µs
§ 8.4.5.2
Modulation, downlink
§ 8.4.1
QPSK, 16-QAM, 64-QAM
§ 8.4.9.4.2
QPSK, 16-QAM
§ 8.4.9.4.2
Convolutional Coding and Convolutional Turbo
Coding
§§ 8.4.9.2.1;
8.4.9.2.3
excluding
§ 8.4.9.2.3.5
AES-CCM, AES Key Wrap, 128-bit keys
§ 11.9.14
EAP
§ 11.8.4.2
Privacy key management
PKMv2
§ 7.2.2
Management message integrity
protection
CMAC
§ 7.5.4.4
Modulation, uplink
Forward error correction coding
Encryption
Authentication
12
5.6.1.1.7 LE operation
The non-exclusive nature of the license-exempt (LE) spectrum introduces additional interference and
coexistence issues. Also, regulatory constraints limit the allowed radiated power as well as imposing other
restrictions. The PHY and MAC introduce mechanisms to facilitate the detection, avoidance, and mitigation
of interference among users including specific spectrum users identified by regulation. This includes a
mechanism for regulatory compliance called dynamic frequency selection (DFS).
LE systems identify both victims and sources of interference and coordinate with neighbor systems in the
interference community to proceed with interference prevention procedures. The LE base stations in all
neighboring systems within an interference community utilize interference information gathered on a
coexistence control channel to negotiate the proper allocation of shared and dedicated channel access that
minimizes co-channel interference. This negotiation uses subscriber stations to forward messages between
the base stations, specifyingcoexistence primitives forcommunication over the infrastructure network.
The coexistence control channel is based on a series of globally synchronized time slots, divided into four
subchannels dedicated for coexistence functions such as passive and active scanning of the radio
environment, interferer identification, and interference resolution.
5.6.1.1.8 WirelessMAN-Advanced
An overview of the WirelessMAN-Advanced radio interface technology is provided in Annex 2 of
Recommendation ITU-R M.[IMT.RSPEC].
5.6.1.2
FDD component
5.6.1.2.1 Introduction
The IEEE standard relevant for IMT-2000 OFDMA TDD WMAN, designated as IEEE Std 802.16, is
developed and maintained by the IEEE 802.16 Working Group on Broadband Wireless Access. It is
published by the IEEE Standards Association (IEEE-SA) of the Institute of Electrical and Electronics
Engineers (IEEE).
The radio interface technology specified in IEEE Standard 802.16 is flexible, for use in a wide variety of
applications, operating frequencies, and regulatory environments. IEEE 802.16 includes multiple physical
layer specifications, one of which is known as WirelessMAN-OFDMA. OFDMA TDD WMAN is a special
case of WirelessMAN-OFDMA specifying a particular interoperable radio interface. The component of
OFDMA TDD WMAN defined here operates in FDD mode.
The OFDMA TDD WMAN radio interface comprises the two lowest network layers – the physical layer
(PHY) and the data link control layer (DLC). The lower element of the DLC is the medium access control
layer (MAC); the higher element in the DLC is the logical link control layer (LLC). The PHY is based on
orthogonal frequency division multiple access (OFDMA) suitable for use in a 5 MHz, 10 MHz or 7 MHz
channel allocation. The MAC is based on a connection-oriented protocol designed for use in a point-tomultipoint configuration. It is designed to carry a wide range of packet-switched (typically IP-based) services
while permitting fine and instantaneous control of resource allocation to allow full carrier-class QoS
differentiation. In addition, a multihop relay specification details the functionality of relay stations in
conjunction with multihop relay base stations in order to provide additional coverage or performance
advantage.
As a technological evolution of OFDMA TDD WMAN, WirelessMAN-Advanced has been introduced for
providing the performance improvements necessary to support future advanced services and applications for
next generation broadband mobile communications. WirelessMAN-Advanced incorporates innovative
communications technologies such as multi-user MIMO (MU-MIMO), multicarrier operation, and
cooperative communications, as well as femto-cells, self-organizing networks, and relays. It also supports
scalable bandwidth operation up to 20 MHz.
5.6.1.2.2 Radio access network architecture
The OFDMA TDD WMAN radio interface is designed to carry packet-based traffic, including IP. It is
flexible enough to support a variety of higher-layer network architectures for fixed, nomadic, or fully mobile
use, with handover support. It can readily support functionality suitable for generic data as well as
13
time-critical voice and multimedia services, broadcast and multicast services, and mandated regulatory
services.
The radio interface standard specifies Layers 1 and 2; the specification of the higher network layers is not
included. It offers the advantage of flexibility and openness at the interface between Layers 2 and 3 and it
supports a variety of network infrastructures. The radio interface is compatible with the network
architectures defined in ITU-T Recommendation Q.1701. In particular, a network architecture design to
make optimum use of IEEE Standard 802.16 and the OFDMA TDD WMAN radio interface is described in
the “WiMAX End to End Network Systems Architecture Stage 2-3”, available from the WiMAX Forum2.
The protocol layering is illustrated in Fig. 70. The MAC comprises three sub-layers. The service-specific
convergence sublayer (CS) provides any transformation or mapping of external network data, received
through the CS service access point (SAP), into MAC service data units (SDUs) received by the MAC
common part sublayer (CPS) through the MAC SAP. This includes classifying external network SDUs and
associating them to the proper MAC service flow identifier (SFID) and connection identifier (CID). It may
also include such functions as payload header suppression (PHS). Multiple CS specifications are provided
for interfacing with various protocols. The internal format of the CS payload is unique to the CS, and the
MAC CPS is not required to understand the format of or parse any information from the CS payload.
The MAC CPS provides the core MAC functionality of system access, bandwidth allocation, connection
establishment, and connection maintenance. It receives data from the various CSs, through the MAC SAP,
classified to particular MAC connections.
QoS is applied to the transmission and scheduling of data over the PHY.
The MAC also contains a separate security sublayer providing authentication, secure key exchange, and
encryption.
Data, PHY control, and statistics are transferred between the MAC CPS and the PHY via the PHY SAP
(which is implementation specific).
The 802.16 devices can include Mobile Stations (MS) or Base Stations (BS). As the 802.16 devices may be
part of a larger network and therefore would require interfacing with entities for management and control
purposes, a Network Control and Management System (NCMS) abstraction has been introduced in this
standard as a “black box” containing these entities. The NCMS abstraction allows the PHY/MAC layers
specified in 802.16 to be independent of the network architecture, the transport network, and the protocols
used at the backend and therefore allows greater flexibility. NCMS logically exists at BS side and MS side of
the radio interface, termed NCMS(BS) and NCMS(MS), respectively. Any necessary inter-BS coordination
is handled through the NCMS(BS).
This specification includes a Control SAP (C-SAP) and Management SAP (M-SAP) that expose control
plane and management plane functions to upper layers. The NCMS uses the C-SAP and M-SAP to interface
with the 802.16 entity. In order to provide correct MAC operation, NCMS shall be present within each MS.
The NCMS is a layer independent entity that may be viewed as a management entity or control entity.
General system management entities can perform functions through NCMS and standard management
protocols can be implemented in the NCMS.
5.6.1.2.2.1
BS and MS functionality
The system architecture consists of two logical entities, the base station (BS) and the mobile station (MS),
with an optional relay station (RS). The basic architectural assumption is of a base station (BS)
communicating in point-to-multipoint fashion with a number of fixed or mobile stations (MSs). The BS is
connected to an IP-based backhaul network. It controls and allocates the resources in spectrum and time.
Transmissions on the downlink (BS to MS) are divided in both time and frequency (using the multiple
sub-carriers provide by OFDMA) for assigning communications to individual MSs. Transmissions on the
uplink (from MS to BS) take place according to the schedule and in the sub-channels assigned by the BS.
In brief, the BS is responsible for:
2
http://www.wimaxforum.org/technology/documents/.
14
–
configuring and updating basic parameters;
–
performing bandwidth allocation for DL (per connection) and UL traffic (per MS) and performing
centralized QoS scheduling, based on the QoS/service parameters and the active resource requests
from the MS;
–
communicating to all MSs, through the maps, the schedule of each frame and supporting other data
and management broadcast and multicast services;
–
transmitting/receiving traffic data and control information as MAC protocol data units (PDUs);
–
–
performing connection admission control and other connection management functions;
providing other MS support services such as ranging, clock synchronization, power control, and
handover.
The MS is responsible for:
–
identifying the BS, obtaining MAC parameters, and joining the network;
–
establishing basic connectivity, setting up additional data and management connections, and
negotiating any optional parameters as needed;
–
generating resource requests for connections that require them, based on the connection profiles and
traffic;
–
receiving broadcast/multicast PDUs and unicast PDUs and forwarding them appropriately;
–
making local scheduling decisions based on the current demand and history of resource
requests/grants;
transmitting only when instructed by the BS to do so or the MS has some information that qualifies
for transmission in one of the allowed contention slots;
–
15
–
unless in sleep mode, receiving all schedule and channel information broadcast by the BS and
obeying all medium access rules;
–
performing initial ranging, maintenance ranging, power control, and other housekeeping functions.
Figure 70 is limited to describing a system including a BS and the MSs with which it communicates.
However, the radio interface also provides specifications to allow handover of an MS from one BS to
another. Such handover would typically occur as a mobile device moves toward an adjacent cell. However, it
might also occur due to system-wide efforts at load balancing.
5.6.1.1.2.2
RS functionality
Multihop relay (MR) is an optional deployment that may be used to provide additional coverage or
performance advantage in an access network. In MR networks, the BS may be replaced by a multihop relay
BS and one or more relay stations (RSs). Traffic and signaling between the MS and the multihop relay BS
are relayed by the RS, thereby extending the coverage and performance of the system in areas where RSs are
deployed. Each RS is under the supervision of a multihop relay BS. In a system providing more than two
hops, traffic and signaling between an access RS and the multihop relay BS may also be relayed through
intermediate RSs.
5.6.1.2.3 Layer 1: Physical layer (PHY)
The radio interface is a special case of the Wireless MAN-OFDMA air interface specified in § 8.4 of IEEE
Standard 802.16. It uses orthogonal frequency-division multiple access (OFDMA), which is an extension of
orthogonal frequency-division multiplexing (OFDM).
5.6.1.2.3.1
OFDMA technology overview
OFDM divides the channel by frequency into orthogonal sub-carriers. Data to be transmitted is divided into
parallel streams of reduced data rate (and therefore longer symbol duration) and each stream is modulated
and transmitted on a separate sub-carrier. The lengthened symbol duration improves the robustness of
OFDM to delay spread. Furthermore, the introduction of a cyclic prefix (CP) eliminates intersymbol
interference if the CP duration is longer than the channel delay spread.
In a typical OFDM implementation, all of the transmitter’s sub-carriers are, at any given time, addressed to a
single receiver; multiple access is provided solely by TDMA time slotting. OFDMA, however, divides the
sub-carrier set into subsets, known as sub-channels. Each sub-channel can
address a different receiver at any given time. In the downlink, each sub-channel may be intended for a
different receiver or group of receivers. In the uplink, multiple MSs may transmit simultaneously as long as
they are assigned different sub-channels.
Sub-carriers are used for three purposes:
–
Data transmission.
–
Pilot transmission, for various estimation purposes.
–
Null transmission, for guard bands and at DC.
The concept is illustrated in Fig. 71. As indicated, the sub-carriers forming one sub-channel need not be
adjacent.
Sub-channelization is a multiple access technique. It provides OFDMA systems increased scheduling
flexibility and a number of performance advantages, including enhanced scalability and advanced antenna
array processing capabilities.
5.6.1.2.3.2
OFDMA TDD WMAN physical layer details
The PHY utilizes OFDMA with either 512 sub-carriers in a 5 MHz channel or 1 024 sub-carriers in a 7 MHz
or 10 MHz channel. The primitive PHY parameters for FDD mode are listed in Table 10B.
16
TABLE 10B
OFDMA TDD WMAN primitive PHY parameters, FDD mode
FFT size (NFFT)
System channel bandwidth (BW)
Sampling frequency (Fs)
512
1 024
1024
5 MHz
10 MHz
7 MHz
5.6 MHz
11.2 MHz
8 MHz
Sub-carrier frequency spacing (∆f = Fs/NFFT)
10.9375 kHz
7.8125 kHz
Useful symbol time (Tb = 1/∆f)
~91.43 µs
128 µs
Guard (CP) time (Tg = Tb/8)
~11.43 µs
16 µs
OFDMA symbol duration (Ts = Tb + Tg)
~102.9 µs
144 µs
Frame duration
5 ms
5 ms
OFDMA symbols per frame
~48
~34
5.6.1.2.3.3
Framing and sub-channelization
The FDD frame structure is illustrated in Fig. 72B. This frame structure can concurrently supports both Full
Duplex FDD (F-FDD) and Half Duplex FDD (H-FDD) Mobile Stations. The frame is partitioned using
MAP1 and MAP2 control structures for H-FDD Mobile Stations.
FIGURE 72B
Schematic illustration of FDD frame structure
Time
OFDMA frame duration
UL frame
TTG2
TTG1
MAP1
Preamble
DL 2
Dlresidue
MAP2
DL 1
DL gap
MAP1
Next frame
DL subframe 2
RTG1
RTG2
UL 2
Ulgap
Frequency
DL frame
Preamble
DL subframe 1
UL 1
M.1457-72b
For systems that serve only F-FDD MSs, the frame structure is configured by allocating the whole down link
and uplink frames to the F-FDD MSs without partitioning of frames.
5.6.1.2.3.4
Adaptive modulation and coding
OFDMA TDD WMAN supports a variety of modulation and coding alternatives. The control is adaptive and
dynamic, so that the BS may select different options for communicating with different MSs and may order
the MS to alter the choices in order to optimize the trade-off of robustness versus capacity.
The BS selects the modulation from among QPSK, 16-QAM, and 64-QAM. For forward error correction,
Convolutional Coding and Convolutional Turbo Coding with variable code rate and repetition coding are
specified. Block Turbo Code and Low Density Parity Check Code (LDPC) are supported as optional
features.
17
Data randomization is specified in order to reduce the peak-to-average power ratio. Interleaving is specified
to increase frequency diversity.
5.6.1.2.3.5
Fast feedback and hybrid ARQ
OFDMA TDD WMAN specifies an uplink fast-feedback channel to provide time-critical PHY parameter
data to the BS. Parameters include signal-to-noise ratio, MIMO coefficients, and MIMO configuration
parameters.
Additional UL acknowledgment channels may be allocated by the BS to support hybrid automatic repeat
request (HARQ).
5.6.1.2.4 Layer 2: Medium access control layer
The medium access control layer (MAC) functionality controls access to the medium, which in this case is
the radio spectrum. The MAC is also responsible for basic functions such as data encapsulation,
fragmentation, radio resource control, radio link control, error detection and retransmission, QoS, security,
sleep mode, and handover.
Although the radio interface is designed primarily to support a connectionless network layer, such as IP, the
MAC is connection-oriented. All services, including inherently connectionless services, are mapped to a
connection. The connection provides a mechanism for requesting resource allocation, associating QoS and
traffic parameters, transporting and routing data, and all other actions associated with the terms of the
service. A 16-bit connection identifier (CID) is assigned to designate each connection. The MAC uses the
CID to identify all information exchanged between BS and MS, including management and broadcast data.
The CID provides a simple and direct way to differentiate traffic. All MAC-level QoS functions, such as the
classifier and QoS scheduler, use the CID to identify and differentiate traffic in order to maintain the service
level and fairness among connections.
5.6.1.2.4.1
Convergence sub-layer (CS)
At the transmitter side, the Convergence sub-layer is responsible for transforming packet-based protocol data
units from the higher layer protocol into MAC service data unit (SDUs), possibly using payload header
suppression (PHS) to suppress some of the packet headers and reduce the burden of carrying them over the
air. The CS then classifies each MAC SDU, assigning it to a particular connection, and passes it to the MAC
CPS. At the receiver side, the CS is responsible for the inverse operations, including reassembly of packets
into their original format with complete headers.
The CS contains a classification function that determines on which connection a particular packet shall be
carried and which PHS rule applies for that packet. The operation is illustrated in Fig. 73, which shows the
downlink case. Classifier parameters are configured during dynamic service signalling.
5.6.1.2.4.2
MAC common part sub-layer
The MAC common part sub-layer (CPS) is responsible for performing the core MAC functions. It receives
MAC service data units (SDUs) from the CS and encapsulates them in its native MAC PDU format for
transmission over the PHY. The MAC CPS also manages the transport connections and QoS, controlling
access to the radio spectrum by the MSs.
Encapsulation may be as simple as adding necessary information to the SDU. However, the MAC CPS also
has the possibility of dividing a single SDU into multiple fragments before transmission, for reassembly at
the receiving MAC CPS. Fragmentation allows more efficient support of higher layer protocols with
variable-size SDUs, given that the underlying PHY used a fixed frame size. The MAC CPS also has the
complementary option to pack multiple higher layer payloads into a single PDU. Since MAC encapsulation
introduces some fixed overhead per PDU, this can improve the efficiency of carrying small SDUs.
A MAC PDU consists of a six-byte MAC header, a variable-length payload, and an optional cyclic
redundancy check. Four header formats, distinguished by the HT field, are defined. The generic header is
shown in Fig. 74.
18
MAC PDUs generally contain either MAC management messages or convergence sub-layer data. However,
one header type is reserved for uplink PDUs that contain no payload, conveying their information (such as a
resource request) in the content of the header itself. Additional sub-headers are also defined. For example,
the MS can use the grant management sub-header to convey bandwidth management needs to the BS. The
fragmentation sub-header contains information that indicates the presence and orientation in the payload of
any fragments of SDUs. The packing subheader is used to indicate the packing of multiple SDUs into a
single PDU. The grant management and fragmentation sub-headers may be inserted in MAC PDUs
immediately following the generic header if so indicated by the Type field. The packing sub-header may be
inserted before each MAC SDU if so indicated by the Type field.
5.6.1.2.4.2.1
Uplink scheduling services
The scheduling algorithm is not specified in the standard but is critical to efficient multimedia delivery when
the BS supports a variety of disparate connections. The BS is presumed capable of scheduling its own
downlink transmissions based on QoS information developed in the CS. Uplink scheduling is more
complicated because, while resource allocation is under the control of the BS, only the MSs know in real
time their immediate transmission demands.
In order to allow efficient QoS-based scheduling, a number of uplink scheduling services are defined, with a
specific service assigned to each connection. The QoS categories are summarized in Table 11.
Resource requests, for transmission slots, are initiated by a specific connection at the MS. However, grants
are allocated not to the connection but to the supporting MS. The MS is required to manage the slots
allocated to it, assigning them to the multiple connections it supports. By distributing the management and
permitting local resource allocation, over-the-air negotiation is minimized and rescheduling decisions are
made more quickly and effectively.
5.6.1.2.4.2.2
Radio link control
As noted in § 5.6.1.3.2, OFDMA TDD WMAN supports adaptive modulation and coding. The MAC CPS is
responsible for radio link control. This involves managing the modulation and coding selection at the MS
through interactive message exchange based on monitoring the ratio of carrier signal to noise and
interference.
TABLE 11
OFDMA TDD WMAN Uplink scheduling services
QoS category
Typical applications
QoS specifications
UGS
Unsolicited grant service
VoIP
Maximum sustained rate
Maximum latency tolerance
Jitter tolerance
rtPS
Real-time packet service
Streaming audio or video
Minimum reserved rate
Maximum sustained rate
Maximum latency tolerance
Traffic priority
ErtPS
Extended real-time packet service
Voice with activity detection (VoIP)
Minimum reserved rate
Maximum sustained rate
Maximum latency tolerance
Jitter tolerance
Traffic priority
nrtPS
Non-real-time packet service
File transfer protocol (FTP)
Minimum reserved rate
Maximum sustained rate
Traffic priority
BE
Best-effort service
Data transfer, web browsing, etc.
Maximum sustained rate
Traffic priority
19
5.6.1.2.4.2.3
Energy conservation in the MS
The MAC CPS controls two energy-saving modes – Sleep Mode and Idle Mode – to conserve energy in the
MS. During Sleep Mode, the MS observes pre-negotiated periods without transmission. Idle Mode is
intended as a mechanism to allow the MS to become periodically available for DL broadcast messaging
without registration at a specific BS as the MS traverses an air link environment populated by multiple BSs,
typically over a large geographic area.
5.6.1.2.4.2.4
Handover
The MAC CPS supports optimized hard handover.
5.6.1.2.4.3
Security sub-layer
The security sub-layer, which operates between the PHY and the MAC CPS, is responsible for providing
strong encryption, decryption, mutual authentication, and secure key exchange. Security is maintained as a
separate sub-layer so that it may be upgraded as necessary. Also, the key functionality internal to the
sub-layer is also modular, to provide easy maintenance upgrade. For example, the protocol provides a means
of identifying one from a set of supported cryptographic suites, each of which specifies data encryption and
authentication algorithms, and the rules for applying those algorithms to a MAC PDU payload.
The security sub-layer utilizes a security association (SA), which is a set of information shared between the
transmitter and receiver. Each SA contains information on the cryptographic suite used for that SA and may
also contain keys, such as the traffic encryption keys (TEKs), along with the key lifetimes and other
associated state information. Prior to transmission, the MAC PDUs are mapped to an SA. The receiver uses
the CID to determine the correct SA and applies the corresponding processing to the received PDU.
Device and user authentication use the IETF EAP protocol. OFDMA TDD WMAN encrypts user data using
the AES-CCM cryptographic suite, with the Advanced Encryption Standard (AES) algorithm in the counter
with CBC-MAC (CCM) mode, with 128-bit keys. The keys are generated using EAP authentication and
managed by a Traffic Encryption Key (TEK) state machine. MAC management messages are AES encrypted
and authenticated. A three-way handshake scheme is supported to optimize re-authentication during
handover.
5.6.1.2.5 Smart antennas
OFDMA TDD WMAN specifies the use of smart antenna technologies, including antenna beamforming,
space-time coding, and spatial multiplexing, which increase the cell size, data throughput, and spectral
efficiency. These techniques reduce the sensitivity of the system to fading and multipath transmission
effects.
5.6.1.2.6 Summary of major technical parameters
TABLE 12B
OFDMA TDD WMAN parameters and capabilities, FDD mode
Parameter/Capability
Duplex method
Physical layer mode
System channel bandwidth
(Uplink/Downlink)
FFT size
Frame duration
Modulation, downlink
Modulation, uplink
Value
IEEE 802.16
Subclause
FDD
§ 8.4.4
OFDMA
§ 8.4
5 MHz
10 MHz
7 MHz
512
1 024
1 024
§ 8.4.1
5 ms
QPSK, 16-QAM, 64-QAM
§ 8.4.5.2
QPSK, 16-QAM
§ 8.4.9.4.2
20
TABLE 12B (end)
Value
IEEE 802.16
Subclause
Convolutional coding and convolutional turbo
coding
§ 8.4.9.4.2
AES-CCM, AES Key Wrap, 128-bit keys
§§ 8.4.9.2.1;
8.4.9.2.3
excluding
§ 8.4.9.2.3.5
EAP
§ 11.9.14
Privacy key management
PKMv2
§ 11.8.4.2
Management message integrity
protection
CMAC
§ 7.2.2
Parameter/Capability
Forward error correction coding
Encryption
Authentication
5.6.1.2.7 WirelessMAN-Advanced
An overview of the WirelessMAN-Advanced radio interface technology is provided in Recommendation
ITU-R M.[IMT.RSPEC].